To make a page asynchronous in earlier versions of ASP.NET, the page has to implement the interface IHttpAsyncHandler and define concrete definitions to all the methods declared in the interface. This takes considerable amount of time and effort to make the page asynchronous.

In ASP.NET 4.5, we can turn the execution of any operation asynchronous by using async and await keywords. Any new page added to an ASP.NET application is considered synchronous by default. We can change this by setting value of Async property of the Page directive to true. Once this property is set, we can use async and await keywords in any method in the code behind file.

This is because, performing asynchronous operations at wrong time may lead to some dangerous conditions on the server. By setting the Async attribute of the page, we are telling the server that, the current page is a safe place to perform async operations. Following is a sample page load event handler that calls a WCF service asynchronously and binds data to a GridView:

The advantage of performing async operation this way over what we did earlier is that, it registers an asynchronous handler with the page and now the execution is not dependent on the synchronization context. Execution of the statements in the PageAsyncTask passed in is not dependent on page the page life cycle event. So, ASP.NET will not wait asynchronously after executing rest of the logic in the life cycle event. It would rather continue execution with the next life cycle event handlers. Statements in the registered task are performed whenever the dependent operation is finished.
Note that, we used the async keyword with a lambda expression. It is legal in .NET 4.5, because lambda is meant to create a single cast delegate with an anonymous method. To make the method executable asynchronously, we can use async and await with lambda.

About Me

I am a software developer working on and passionate about Microsoft Technologies and anything new in the Web world. Lately, spending a lot of time with JavaScript and finding it much deeper than what I knew for past few years. I enjoy writing code and writing about code. My contribution to community earned me a Microsoft MVP award (ASP.NET/IIS) and a DZone MVB award. When not in front of computers, like to spend time with melodious Hindi and Telugu tunes or watching Cricket